Question

What are the input variable and the output variable?

number of rowers in a rowboat and the speed of the rowboat

input: number of rowers in the rowboat; output: speed of the rowboat

input: speed of the rowboat; output: number of rowers in the rowboat

input: number of rowboats; output: speed of the rowboat

input: number of rowers in the rowboat; output: the number of rowboats

Answer 1

The answer is: [A]:
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  input:     number of rowers number of rowers in the rowboat;
  output:   speed of the rowboat ;
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NOTE:  The input would be the independent variable and plotted on the "x-axis" of the graph; and the output would be the dependent variable and plotted on the "y-axis" of the graph.
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Answer 2

According to the above sentence i think that suitable answer would be option A
input: number of rowers in the rowboat; output: speed of the rowboat.